Twenty years ago, most major oil companies sold gasoline in all 50 states. But today, if you live in California you're likely filling up at an Arco or Chevron service station, and if you're visiting Chicago, Amoco and Shell are probably going to get your business. In the Northeast, Mobil, Exxon and Amoco occupy more corners than anyone else.

A trend begun in the 1960s and '70s has accelerated the past couple of years: The major oil companies realized that being everywhere wasn't advantageous for the bottom line. For several reasons, big oil companies are carving up the nation into retail regions that assist strategies to boost refining and marketing profits.

Oil companies say consumers will still see plenty of competition, despite the consolidation. But some consumer advocates believe the long-term result will be higher prices at the pump.

The best example of concentrating on core markets is Atlantic Richfield's decision in the 1980s to pull out of the East Coast and concentrate on five Western states instead of 17 states. The company, which sells more gasoline in the five states than it did in the 17, now commands 26 percent of the Los Angeles market and almost 20 percent of the California market.

This this year, Mobil has said that it is leaving Houston; Exxon has decided to exit Los Angeles; British Petroleum has announced that it is pulling out of California totally; and Chevron has said that it no longer will sell gasoline in Montana and part of Wyoming.

When companies pull out of a region, the stations usually change nameplates. For example, Shell is picking up Mobil's 90 stations in Houston, while Mobil swaps for Shell's Milwaukee stations. Chevron recently completed conversion of 69 stations it purchased last year from Shell in Atlanta. Chevron picked up some of Exxon's Los Angeles stations.

The companies say consumers benefit from the trend because stations are more efficient. Besides, they say, each company still will be operating in a competitive market: Where there were formerly maybe a dozen suppliers, there will now be half a dozen companies as well as several regional marketers seeking the same pool of consumers.

To succeed in a market, you have to have economies of scale or the market share to be efficient, offer good service and price, said Steve Pryor, U.S. marketing manager for Mobil Corp. "If you can do all that, the consumers aren't left out because standards are raised."

"They're cutting up the country, but not in concert," said Trilby Lundberg, publisher of the Lundberg Letter, a statistical and analytical newsletter for the oil marketing industry.

"The areas where companies are pulling out aren't suffering from a lack of fuel" and markets remain competitive, she said. No single major oil company has more than an 8 percent market share nationwide, according to the Lundberg Letter's annual review.

"The majority of trips that people make are within their metro areas," he said. "People do most of their driving within a 50-mile radius of their homes. No one wants to go to war with the dominant, lowest-cost marketer in an area."

Consumer groups focus on price and charge that consolidation is destroying independent refiners and gasoline marketers. Increased market power allows the major companies to quickly raise retail prices after global oil prices jump, as they did after Iraq invaded Kuwait.

The "increased market power" was apparent to consumers during the crude-oil price declines of 1986, 1988 and the first half of 1990, Mr. Rothschild said.

"During these periods, major oil companies did not pass through to motorists the savings realized from lower crude costs," he said. "And during periods of market disruption, in 1989 following the Exxon Valdez disaster and in 1990-1991 following Iraq's invasion of Kuwait, there was no delay in passing through higher prices."

U.S. retail pump prices are among the lowest in the world because other nations heavily tax gasoline. In Europe, prices range from $2.50 to more than $4 a gallon.

Major oil companies are sensitive about their strategies and pricing. Big Oil has been the subject of several congressional hearings and investigations since gasoline and the automobile were invented. Just this month, a subcommittee of the House Energy and Commerce Committee held hearings on competition in the retail gasoline market.

Those hearings didn't make headlines because gasoline prices are relatively low and consumers aren't screaming. According to a study by Dan Yergin, president of Cambridge Energy Research Associates, during the post-World War II era the lowest retail gasoline price was 23 cents a gallon in 1947. Adjusted for inflation and translated in to 1991 dollars, the 1947 pump price equals $1.49 a gallon today.

In 1988, gasoline prices averaged $1.06 a gallon, the lowest average annual price -- in 1991 dollars -- since World War II, according to Mr. Yergin's study.